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Full text of "The manufacture and properties of iron and steel"

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306                         METALLUBGY OF IRON AND STEEL.
'••.ings.:—It has been proven rby many investigators and is generally
-acknowledged that in heating steel through the lowest critical point
.the crystalline structure is obliterated, the metal assuming the
finest condition of which it is. capable.   Above this point the size
of the grain increases with the temperature.   There is a difference
of opinion as to whether the increase in size: takes place during
the .heating or at the moment when cooling begins, but it is un-
•. necessary to determine, this question, the general proposition •being
.true that the higher a piece of steel is heated above this point the
larger the grain becomes.
At the corresponding point in cooling, the structure ceases to
• change, except in very soft steel, as shown by Stead, and any size of grain is retained and cannot be changed by heat treatment below.
• this point.    There is, however, a change from hardening to cement <carbon,-which may take place at comparatively low temperatures. jThis is the principle on which the tempering of steel is founded,
quite' a definite amount, being changed at temperatures which are : represented approximately by the color of the bar. Cement carbon 'is that form which confers the softest possible condition and great-,est ductility, while hardening carbon, gives the condition of greatest hardness. , Hence the temper is drawn by every rise in tempera-"ture. .                                            '•'.,..              i .
At the lowest critical point the change from cement to hardening -carbon takes place almost instantly, all carbon above this tempera-"ture being of the hardening variety, but the reverse change in cool-' ing appears to require a certain length of time.    This is the explanation of hardening' by quenching, the more rapidly the steel is cooled through this point, the less being the chance of the carbon •to  change its  state.    A  sudden cooling in ice water prevents ['any  change,   while   annealing is   effective   only  in  proportion ~ as the time of exposure to this temperature was long or short. Since fine structure and cement carbon are the principal factors of toughness and ductility, both of which are the aim in annealing, it would seem that the best method of tempering would be to heat to .the lowest critical point and not higher, and quench from this heat /and subsequently draw the temper.    Similarly the best way of an-^eajing, since the reverse change takes place several degrees below this, would be to cool at once to just above this lower point and allow several hours for the metal to cool past the critical tempera-